The present invention is directed to subsurface well tools, for example only, safety valves, which utilize a hydraulic piston and cylinder actuator. The piston and cylinder assembly of the present invention has a cross-sectional area exposed to hydraulic fluid in a control line in which the area exposed to the control fluid increases in one direction of movement of the hydraulic assembly and decreases in the opposite direction of movement.
As the setting depths of hydraulically actuated well tools, such as safety valves, continues to increase, the energy required to move the well tool against the hydrostatic head acting on the hydraulic actuator also increases. For example, on conventional safety valves, suitable biasing means, such as a gas chamber or more usually a power spring, acts on the hydraulic actuator to overcome the hydrostatic force. One method of increasing the setting depth is set forth in U.S. Pat. No. 4,161,219 of reducing the hydraulic area of the hydraulic actuator which allows the existing biasing forces to overcome greater hydrostatic heads. However, there are practical limits to maximizing biasing forces such as springs, and minimizing the hydraulic areas of a hydraulic piston and cylinder assembly. Generally, to move a small hydraulic piston and cylinder assembly against a high hydrostatic head requires a strong spring which results in a high "spread" in the operating pressure to move the well tool from a first position to a second position. This increased spread increases the problem of (1) surface operating pressures, (2) the biasing means, such as springs, which require very high pounds of force and length and become quite expensive, and (3) the valve increases in length and expense because of a required longer spring.
The present invention is directed to a subsurface well tool having a hydraulic actuator with a variable hydraulic area. That is, the hydraulic assembly includes a cross-sectional area exposed to hydraulic control fluid in which the area increases in one direction of movement of the assembly and decreases in the opposite direction of movement. This change in hydraulic area, upon movement of the hydraulic actuator, reduces the operating pressure spread thereby reducing surface operating pressure problems, and decreases the size and expense of the biasing means as well as the well tool.